1. Field of the Invention
The present invention relates to a metal support for exhaust gas purifying catalyst, which is configured such that a honeycomb structure is mounted in a cylindrical casing.
2. Description of the Related Art
In related art honeycomb structures of a type in which a metal flat sheet and a metal corrugated sheet overlapped thereon are spirally wound, the metal flat sheet is joined to the metal corrugated sheet using a binder composed of a Ni-based brazing filler metal. Also a method of joining a metal flat sheet to a metal corrugated sheet using a stopper in place of the binder is described in Japanese Patent Application Laid-open No. 5-228376, and a method of joining the end portion of a metal flat sheet to the end portion of a metal corrugated sheet by welding is described in Japanese Patent Publication No. 4-35271.
Incidentally, the joining of a metal flat sheet to a metal corrugated sheet using a binder is allowed to ensure a sufficient joining area, which is effective to increase a mechanical strength; however, it has the following problem: namely, if a metal support composed of a cylindrical casing containing a honeycomb structure in which a metal flat sheet is joined to a metal corrugated sheet using the binder is repeatedly heated and cooled in a real service environment, then there may occur breakage of the metal flat sheet and the metal corrugated sheet and/or peeling of the joining portion, finally leading to looseness of the honeycomb structure from the cylindrical casing.
A honeycomb structure in which a metal flat sheet is joined to a metal corrugated sheet using the stopper described in Japanese Patent Application Laid-open No. 5-228376, mentioned above, exhibits a good durability against repeated heating and cooling; however, it has a problem in terms of durability against vibration because the metal flat sheet is not joined to the metal corrugated sheet, which tends to cause breakage of the metal flat sheet and the metal corrugated sheet. Even if the end portions of the metal flat sheet and the metal corrugated sheet are joined to the stopper by welding or the like in order to avoid the above problem, there occurs another problem in terms of the increased ventilation resistance due to the stopper.
A metal support composed of a cylindrical casing containing a honeycomb structure in which the end portion of a metal flat sheet is welded to the end portion of a metal corrugated sheet as described in Japanese Patent Publication No. 4-35271, mentioned above, is poor in durability against vibration because only the end portions of the metal flat sheet and the metal corrugated sheet are welded to each other, with a result that there may occur breakage of the metal flat sheet and the metal corrugated sheet and/or peeling of the joining portion, finally leading to falling of the honeycomb structure from the cylindrical casing.
To solve the above-described problems, there may be considered a honeycomb structure in which a metal flat sheet is welded to a metal corrugated sheet at a plurality of welding positions arranged on each of mutual contact portions of the metal flat sheet and the metal corrugated sheet.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a honeycomb structure, which is capable of improving a durability against thermal stress of the honeycomb structure and a durability against mechanical vibration of the honeycomb structure by specifying welding positions at the time when a metal flat sheet is welded to a metal corrugated sheet.
To achieve the above object, according to a first aspect and feature of this invention, there is provided a metal support for exhaust gas purifying catalyst, which is configured such that a cylindrical honeycomb structure is formed by spirally winding a strip metal flat sheet and a strip metal corrugated sheet overlapped thereon while welding, for each turn, the metal flat sheet to the metal corrugated sheet at a plurality of welding positions axially arranged on each of mutual contact portions of the metal flat sheet and the metal corrugated sheet; and the honeycomb structure, on the surface of which exhaust gas purifying catalyst is supported, is mounted in a cylindrical casing, characterized in that the maximum value L1 of distances each between adjacent ones of the plurality of welding positions satisfies at least one of relationships given by
L1xe2x89xa6D/2 and L1xe2x89xa6L/2 
where L is the axial length of the honeycomb structure and D is the diameter of the honeycomb structure.
With this configuration, it is possible to prevent occurrence of breakage of the metal flat sheet and the metal corrugated sheet and peeling of weld portions, and hence to improve a durability against thermal stress of the honeycomb structure and a durability against mechanical strength of the honeycomb structure.
According to a second aspect and feature of this invention, there is provided a metal support for exhaust gas purifying catalyst, which is configured such that a cylindrical honeycomb structure is formed by spirally winding a strip metal flat sheet and a strip metal corrugated sheet overlapped thereon while welding, for each turn, the metal flat sheet to the metal corrugated sheet at a plurality of welding positions axially arranged on each of mutual contact portions of the metal flat sheet and the metal corrugated sheet; and the honeycomb structure, on the surface of which exhaust gas purifying catalyst is supported, is mounted in a cylindrical casing, characterized in that the distance L2 between the outermost one of the plurality of the welding positions in the axial direction of the honeycomb structure and the outer end surface of the honeycomb structure satisfies at least one of relationships given by
L2xe2x89xa6D/20 and L2xe2x89xa6L/10 
where L is the axial length of the honeycomb structure and D is the diameter of the honeycomb structure.
With this configuration, it is possible to prevent occurrence of breakage of the metal flat sheet and the metal corrugated sheet and peeling of weld portions, and hence to improve a durability against thermal stress of the honeycomb structure and a durability against mechanical strength of the honeycomb structure.
According to a third aspect and feature of this invention, there is provided a metal support for exhaust gas purifying catalyst, which is configured such that a cylindrical honeycomb structure is formed by spirally winding a strip metal flat sheet and a strip metal corrugated sheet overlapped thereon while laser welding using a laser beam, for each turn, the metal flat sheet to the metal corrugated sheet at a plurality of welding positions axially arranged on each of mutual contact portions of the metal flat sheet and the metal corrugated sheet; and the honeycomb structure, on the surface of which exhaust gas purifying catalyst is supported, is mounted in a cylindrical casing, characterized in that the laser welding, at the outermost one of the plurality of welding positions in the axial direction of the honeycomb structure, is performed by irradiating the outer end surfaces of the metal flat sheet and the metal corrugated sheet with the laser beam; and the laser welding, at each of the remaining positions, is performed by irradiating the circumferential surfaces of the metal flat sheet and the metal corrugated sheet with the laser beam.
With this configuration, it is possible to certainly weld the metal flat sheet to the metal corrugated sheet while avoiding burning-through of the outer end surfaces of the metal flat sheet and the metal corrugated sheet.
Hereinafter, a mode for carrying out the present invention will be described with reference to an embodiment shown in the accompanying drawings.